As security relating to information is increasingly becoming important nowadays, there is significantly increasing demand for the above-mentioned data processing apparatus capable of encrypting data which should not desirably be known to a third party (herein referred to as “subject data”) and recording it on a recording medium. Although various encryption techniques are proposed and put into practical use in order to keep the subject data confidential, it is difficult to completely prevent a code from being broken.
In general, the subject data is encrypted and recorded on a predetermined recording medium by cutting the subject data by a predetermined number of bits and recording encrypted data having encrypted pieces of the cut data connected as one on the predetermined recording medium.
In this case, the data processing apparatus performs encryption or decryption processing in fixed units, such as 8 bits or 16 bits.
Therefore, if such units are known to the outside, there is a higher possibility that an algorithm and key used for the encryption are broken and the contents of the communication are consequently broken.
An existing data processing apparatus performs the encryption or decryption in standardized units, such as 8 bits or 16 bits. Therefore, in the case where the encryption or decryption is performed in the same units as the standardized units, there is a danger that the algorithm and key used for the encryption may be easily broken on another data processing apparatus.
An object of the present invention is to improve a data processing system including two communication apparatuses capable of encrypting the subject data in plain text to render it as encrypted data and then transmitting it to the communication apparatus at the other end and also decrypting received encrypted data and rendering it as the subject data so as to reduce the possibility of having communication broken by a third party.